The present invention relates generally to a continuously variable transmission device, and particularly to such a device in which forces are transmitted by rolling traction.
One known type of continuously variable transmission, which has been put into commercial use in the drive train of a motor vehicle comprises a transmission belt of trapezoidal section passing around two split pulleys the opposite halves of which have facing inclined conical surfaces and are urged towards one another such that the effective transmission ratio between one pulley and the next is determined by the relative separation of the two parts of the pulleys. On being moved apart, the two parts of a pulley allow the belt to contact the conical faces at a radially inner position thereby changing the transmission ratio.
In this case the resilient forces exerted on the two parts of the other pulley cause them to move together to compensate for the reduction in tension in the belt thereby increasing the radius of the contact between the belt and the pulleys. Such transmission devices are, however, relatively large as they comprise two substantially spaced shafts, and moreover are not capable of rapid ratio change. They also tend to have low efficiency under part load operation. Versions having rubber belts can also be subject to large amounts of wear largely because of the sliding friction which takes place between the belt and the pulleys as the gear xe2x80x9cratiosxe2x80x9d are changed. It does have the advantage over conventional gear boxes of providing a continuous or stepless variation in drive transmission from a minimum to a maximum drive ratio. It is not, however, capable of providing a reverse ratio without further structural complications.
Other forms of continuously variable transmission are known, for example from U.S. Pat. No. 1,800,388. The present invention seeks to provide a continuously variable transmission in which the forces are transmitted between the members in motion by rolling contact. This minimises the amount of wear to which the members are subject upon changing the transmission ratio, and allows a continuously variable transmission to be made which is easy to control, requires low maintenance and offers a durable mechanism with a long service life.
According to one aspect of the present invention, therefore, there is provided a continuously variable transmission device of the type having planet members in rolling contact with radially inner and outer races each comprising two axially spaced parts, with control means for selectively varing the axial separation of the two parts of one race and thus the radial position of the planet members in rolling contact therewith, in which there are provided means sensitive to the torque applied to a drive-transmitting member of the transmission operable both to determine the compensating variation in the separation of the two parts of the other race and thus the transmission ratio of the device and to vary the forces exchanged between the planets and the races normal to the interface between them.
In a preferred embodiment of the invention the planet members are substantially spherical bodies. They may be right circular, oblate or prolate spheroids. Alternatively, the planet members may have respective first and second surface portions comprising surfaces of revolution about the same axis (for each member) the surface portions being inclined with respect to one another in opposite directions about the axes of revolution. The planet members may have a convex or concave surface of revolution defined by a curved generatrix which may be a regular or irregular curve or a part-circular curve. In the case of a part-circular generatrix this may be a semi-circle, in which case the surface of revolution of the planet member is spherical.
In such a structure the inner and outer races preferably comprises two parts, one in contact with each of the first and second portions respectively, and each having a respective surface constituted by a surface of revolution about a common axis and inclined in opposite directions with respect to the said axis. The two parts of one of the inner or outer races may be supported in such a way as to be relatively displaceable towards or away from one another whereby to vary the radius of the point or line of contact between the said one race and the planet members.
Embodiments of the invention may be provided with torque-sensitive mechanical coupling means interposed between an input drive member and one of the races whereby to balance the torque transmission and the contact pressures between the two parts of that race and the planet members.
In practice, it is preferred that the planet members are substantially spherical and captive between the radially inner races and the radially outer races, there being roller follower members circumferentially intercalated between adjacent pairs of planet members for transmitting drive to or from the said planet members. In such an arrangement it is particularly convenient if the roller follower members are carried on a planet carrier member to which drive to or from the planet members is transmitted in operation of the device.
In general terms, the present invention provides a drive transmission device as defined above, in which the axes of rotation of the planet members about their own axis are substantially parallel to the axis of rotation of the planets about the radially inner race. In such a drive transmission device it is a particular feature that the axis of rotation of the radially inner race is substantially parallel to the axis of the means defining the radially outer race defining the planetary path of the planet members.
In any event it is convenient if the means for selectively varying the axial separation of the two parts of the radially inner race or the means defining the radially outer race include two adjustment members interconnected by a helical interengagement such that relative turning motion of one of the adjustment members results in relative axial displacement of the other. In such a device the helical interengagement of the two adjustment members comprises a screw threaded engagement of the members themselves, the said one of the two adjustment members being turnable through at least a limited arc of movement about a first axis and the said other of the adjustment members being restrained against rotary motion at least about an axis substantially parallel to the said first axis. The helix angle may be constant over the entire length of the helix although for certain applications it may be found to be useful if the helix angle of the said helical interengagement varies over the circumferential extent of the helix.
In general, it is convenient if the said other of the two adjustment members is or is carried by or on the said means defining the radially outer track. The two parts of the radially inner race may be carried on a drive or driven shaft, and the means for allowing relative separation of the two parts of the radially inner race comprise at least one inclined surface acting to react the forces exerted by the transmission of drive forces between the radially inner race and the planet members. The said inclined surface may be part of a helical interengagement between the parts of the radially inner race and the drive shaft. The helix angle may be constant or may vary over the length of the helix.
Whether the helix is constant or varying its form and helix angle should preferably be such that the circumferential component of the axial force reacted by the helix is substantially equal to and opposite in sign from the direct circumferential force reacted by the helix such that the force required to be applied to the said selective adjustment means to maintain or change a transmission ratio is minimised.
The present invention also comprehends an infinitely variable transmission device comprising a continuously variable transmission as defined above together with a further epicyclic transmission train of fixed ratio gears or rolling traction members the dimensions of which are such that the effective radius of contact between the gears about the axes of the input shaft lies at a radius between the maximum and minimum radius of the line of contact between the radially inner race and the planets at opposite ends of the range of adjustment of the continuously variable transmission.
In such an embodiment when the radius of the line of contact between the planet members and the radially inner race of the continuously variable transmission is equal to the contact radius between the radially inner race and the planets of the epicyclic drive train of fixed ratio gears there is no effective transmission of torque and the torque transmission is delivered in one rotary direction or the other depending on whether the adjustment of the continuously variable transmission moves the line of contact between the planet members and the radially inner race to a point greater than or less than the radius of contact of the fixed ratio epicyclic gear train. This makes it possible to provide both forward and reverse transmission ratios.
In another aspect the present invention provides a rolling contact continuously variable transmission device of the type having planetary members in rolling contact with radially inner and outer races each comprising two relatively axially displaceable race parts, in which the planetary members are substantially spherical and the transmission forces to or from the spherical planetary members in planetary motion is effected via roller follower members.
In a practical embodiment the roller follower members are each interposed between respective pairs of adjacent planetary members and carried on a planet carrier member through which drive to or from the planet members is transmitted in operation of the device.
The rolling contact continuously variable transmission device of the invention may be considered, in one aspect, as a variable geometry four point contact rolling element bearing in which power transfer takes place between two or three principal bearing elements comprising a radially inner race, a radially outer race and a planet carrier or cage; a fourth bearing element being provided, usually fixed, for torque reaction.
In a transmission device formed as an embodiment of the invention the axial separation of the race which compensates for adjustment of the other is therefore determined in essence by the forces applied to the said other race elements. Such a continuously variable transmission may be combined with an epicyclic gear train to provide an infinitely variable transmission which has a transmission ratio varying from a negative value or nil to a maximum value determined by the dimensions of the device. It is also possible so to chose the relative shapes of the contacting surfaces of the races and the rolling elements that a so-called xe2x80x9cgearedxe2x80x9d neutral position can be achieved in which no transmission of motion takes place despite the rotation of the drive member. Such a configuration also, therefore, allows the rolling elements to be in such neutral position at an intermediate point in their overall range of movement (which movement is determined by the controlled separation of that pair of the race members to which input control forces are applied) thereby allowing relative rotation of the input and output drive members in the same or in opposite directions depending on the adjustment of the transmission. This effectively results in the provision of forward and reverse drive ratios on either side of a neutral ratio.
Although such transmissions can be controlled in such a way that infinitely variable transmission ratio control can be effected, such control is unfamiliar to the majority of users in view of the almost universal use of stepped or incremental drive transmission ratios available from gearboxes used for effecting such ratio changes. Embodiments of the present invention can be made in which incremental control of the gear ratios is achieved by various means whereby to simulate a stepped gearbox. In one embodiment a control mechanism by which the separation of the two controlling raceways is determined has an incremental adjustment device or indexing mechanism allowing it to be displaced between several discrete predetermined positions. Alternatively, the contacting surface of some of the raceways may be shaped such that the forces exerted on the rolling elements tend to drive them to one of a limited number of predetermined positions.
Although a xe2x80x9cgearedxe2x80x9d neutral has many advantages in a transmission device such as that defined herein inevitable tolerances may result in their being a certain amount of xe2x80x9ccreepxe2x80x9d in either direction when the transmission device is set in its neutral position. To combat this it may be advantageous to provide means by which a xe2x80x9cdisconnectedxe2x80x9d neutral ratio may be achieved in which there is a positive break in the transmission chain allowing certainty in the selection of a neutral gear that no drive transmission will take place.
A disconnected neutral may be achieved in one aspect of the present invention by the addition of a relatively rotatable member or corresponding members, to either the radially inner or radially outer raceway contactable by the rolling elements over a certain part of the range of movement thereof in adjustment of the drive transmission. When the rolling elements are in contact with such relatively rotatable members the effective decoupling of the rolling elements from the raceways ensures that drive transmission does not take place in this adjustment.
A disconnected neutral may also be achieved by mounting one of the parts of the raceway in such a way that it can be withdrawn from its working position by a distance such as to release the pressure on the rolling elements at the time. This allows, in effect, a xe2x80x9cdeclutchingxe2x80x9d action to be achieved with the transmission device set in any gear ratio.
A similar arrangement may be provided for a so-called xe2x80x9claunchxe2x80x9d control that is for progressive engagement of the transmission device from neutral to a drive gear ratio, and this can be achieved effectively by providing a range of motion of a ratio control member between the geared neutral position and a first detent or stop defining a lowermost gear ratio.
In some embodiments of the present invention described so far the drive transmission from an input shaft to an output shaft can only take place in one direction of rotation. This arises because the torque-sensing mechanism, which in one embodiment involves a helical interengagement between one of the two race parts in the said other race and a cooperating component allows the two race parts to be urged towards one another by the forces exerted on them in operation only when the direction of rotation of the input shaft corresponds to that of the helical interengagement. Relative rotation between the input shaft and the output shaft in the opposite direction would result in a relative separation of the other race parts which would effectively result in a reduction in the contact forces and, ultimately, to a decoupling of the input and output members. This, of course, has certain advantages in some circumstances, particularly where an over-run free-wheel effect is desirable. However, for use as a motor vehicle transmission, especially one in which engine over-run is used for braking, the free-wheel effect is unwanted and, indeed, decidedly undesirable.
The present invention also seeks, therefore, to provide a continuously variable transmission device of the type described herein in which the transmission of torque from an input to an output shaft can take place in either direction of rotation.
According to another aspect of the present invention, therefore, there is provided a continuously variable transmission device of the type having planetary members in rolling contact with radially inner and outer races each comprising two axially spaced parts, with control means for selectively varying the axial separation of the two parts of one race and thus the radial position of the planetary members in rolling contact therewith, in which there are provided means sensitive to the torque applied to a drive-transmitting member of the transmission device, operable both to determine the compensating variation in the separation of the two parts of the other race and thus the transmission ratio of the device and to vary the forces exchanged between the planets and the races normal to the interface between them, and in which the said torque-sensitive means include the two axially spaced, relatively moveable parts of the said other race, each said part being itself axially movable in two directional senses from a central position and engagable by limit stop means whereby to allow the transmission of rotary drive from a rotary drive input member to a rotary drive output member of the transmission device in each of two opposite senses of rotation.
In a preferred embodiment of the invention the said relatively movable race parts of the torque-sensitive means are interconnected with the input drive member by a screw-thread engagement of the same hand by which rotary drive is transmitted when axial displacement of a race part is restrained.
The thread flights of the screw thread engagement are preferably interengaged by rolling elements such as balls although this is not essential. The provision of interengaging balls helps significantly to reduce frictional resistance in the device.
The said two relatively movable race parts of the torque-sensitive means may be oppositely axially resiliently biased. This resilient bias act to xe2x80x9cprimexe2x80x9d the torque-sensing reaction of the device and in a preferred embodiment of the invention the resilient biasing of the said two relatively movable race parts is achieved by a compression spring located between them.
Of course, in order to ensure that bi-directional rotation can take place each of the two race parts must ultimately be restrained from axial movement such that the other race part can, effectively xe2x80x9cscrew upxe2x80x9d against it by the helical action exerted on it by the input member. Such limit stop means may comprise respective abutments on or carried by or associated with the said input drive member.
In one embodiment of the invention the two race parts of the said one race of the transmission device, the axial separation of which is selectively variable, are each carried on a casing of the transmission device in such a way as to have a limited rotational displacement in each of two opposite rotational senses. The relative axial separation of the two race parts of the said one race may be achieved by a helical interengagement of at least one of the two race parts with a fixed member of the transmission device, the two race parts both being relatively turnable with respect to the said fixed member. Such relative turning movement of the two race parts of the said one race may be achieved by any means which act directly between them rather than between one member and a fixed part. One means by which this can be achieved comprises a Bowden cable acting between the two race parts.
The present invention also comprehends, independently of the structure allowing bi-directional rotation to be achieved, a continuously variable transmission device of the type having planetary members in rolling contact with radially inner and outer races each comprising two axially spaced parts, with control means for selectively varying the axial separation of the two parts of one race and thus the radial position of the planetary members in rolling contact therewith, in which the planetary members each have a circumferential annular groove the axis of which substantially coincides with the respective rolling axis about which each planetary member turns as it rolls in contact with the races, the said annular grooves being engaged by roller follower members acting to guide the planetary members to maintain their orientation in their planetary motion.
This latter feature enables a greater load-carrying capacity to be achieved because a greater number of planetary members can be arranged in a given annular space because the circumferential space occupied by a planetary member can overlap that occupied by a planet follower.
The planet followers are preferably carried by a common carrier member through which drive transmission is conveyed to an output drive member of the device.
According to a further aspect of the present invention a continuously variable transmission device of the type having planetary members in rolling contact with radially inner and outer races each comprising two axially spaced parts, with control means for selectively varying the axial separation of the two parts of one race and thus the radial position of the planetary members in rolling contact therewith, has planetary members each with arcuately curved surface portions in rolling contact with correspondingly curved portions of the respective races, the radius of curvature of the said surface portions of the planetary members being greater than the effective radius of the planetary member itself.
This can be visualised by imagining the planetary members as spheres of a given diameter notionally split to remove a central portion and reassembled with the remaining quadrants in contact with one another. The radius of curvature of the surface portions will thus match that of the xe2x80x9coriginalxe2x80x9d sphere whilst the diameter of the newly-assembled sphere will be less than the diameter of the original sphere. Such planets may also be formed with circumferential grooves for receiving roller follower guide members as discussed above. There may further be provided means for guiding the planetary members to maintain the orientation of their rolling axes as they roll over the contacting surfaces of the races. Such guide members may be the above-mentioned rollers engaged in the circumferential grooves.
The purpose of enlarging the radius of curvature of the surface portions in relation to the diameter of the planetary member itself, is to extend the range of ratios which can be transmitted by the transmission device. In a specific embodiment, which will be described in more detail hereinbelow, the ratio range can be extended to 4.3:1.
In a further aspect of the present invention, which may be considered independently of the other aspects described hereinabove, there is provided a continuously variable transmission device of the type having planetary members in rolling contact with radially inner and outer races each comprising two axially spaced parts, with control means for selectively varying the axial separation of the two parts of one race and thus the radial position of the planetary members in rolling contact therewith, in which each planetary member has a plurality of elementary annular contact surface portions having a substantially constant inclination to the rolling axis of the planetary member itself.
This allows the continuously variable transmission to be provided with preferred adjustment positions effectively representing specific gear ratios of a conventional gear box. Increased load-bearing capacity is also achieved by providing what amounts to a line rather than a point contact between the planets and the races over the surface portions having substantially constant inclinations.
This can be viewed as a planetary member having a generatrix which includes a section comprising a plurality of substantially rectilinear elementary portions. The races may have substantially continuously curved contact surfaces or may have respective contact surfaces for rolling contact with the planetary members, each having correspondingly inclined elementary annular contact surface portions substantially matching those of the planetary members.